Methods and apparatus for developing forces with ion beams



Aug. 21, 1962 G. c. BALDWIN METHODS AND APPARATUS FOR DEVELOPING FORCES WITH ION BEAMS Filed Aug. 12, 1960 s m .n u m w e w r n o d z s O r a C c x a O 6 )6 N m N w B t m r m m n 6 w m w m e A 0 D 0 0 V. e 4 5 n n .v WW 11111 5 |1||||| A r 1C a a 1111 11 QRENSK dv fi m wq k 5 5 WNW United States atet 3,050,652 METHODS AND APPARATUS FOR DEVELOPING FORCES WITH ION BEAMS George C. Baldwin, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Aug. 12, 1960, Ser. No. 49,366 7 Claims. (Cl. 31363) My invention relates to force developing systems and, in particular, to methods and apparatus employing a beam of high speed ions to develop a force for propulsion purposes.

Ion propulsion is a means for generation of reaction thrust in which electrostatic fields impart momentum to ions of an expellant substance. The expellant must be ionized, its ions and electrons separately accelerated to the appropriate velocities by independent sets of electrostatic fields fixed in the vehicle, remixed and exhausted from the vehicle in more or less well collimated fashion. The vector sum of the momenta of all particles permanently leaving the vehicle per unit time is the average thrust produced in that time interval.

The propulsion of a vehicle in space utilizes the re action force mv on the vehicle due to ejection of a mass m of expellant at velocity v and mass fiow rate m. In order to accomplish a given mission, a definite increment of momentum mv must thereby be imparted to the expellant. In order to conserve expellant mass, it is desirable to make the exhaust velocity v as high as possible. Since v is limited for chemical propellants, electrical propulsion schemes are of considerable current interest. These involve the ionization of the expellant and its acceleration by electric fields. It is necessary, however, that the vehicle maintain electrical neutrality. This requires that the ejected ion beam be neutralized by the mixture of an equal current of electrons. Attempts to introduce electrons into accelerated ion beams have proved diflicult because of the high electric fields created by the ion space charge. These fields cause the electron to execute intense oscillations which preclude the matching of electron and ion velocity and constitute a severe energy drain upon the vehicle power supply.

When an ion beam is used as the force developing system for propelling a vehicle, as a consequence of the electrical isolation and the low intrinsic capacitance of the system, the potential of the system will change rapidly if any net current is drawn causing it to interact with the beam expelled. This potential change may rapidly become sufiicient to stop all net charge flowing from the system during a time interval corresponding to a very short motion of the expellant particles. Described in another way, for each ion ejected, an electron remains lowering the vehicle potential and increasing the field about the vehicle which retards the ions. It is, therefore, imperative that electrons be emitted along with the ions, and it has been recognized that it is desirable to mix such electrons with the ion beam in the interest of efficiency. The equality of ions and electrons in the current then guarantees that steady emission is at least on the average possible.

In an ion propulsion system, therefore, it is also necessary that the total electron current exactly match the total ion current if the vehicle is not to acquire a charge. So far as the vehicle potential is concerned, electrons could be emitted at some part of the vehicle other than the point of emission of the ions and need not match the ion density or velocity. This, however, not only is a difiicult feat to perform, but moreover the capacitively stored energy of the ion beam constitutes an unwanted drain on the power source. More importantly, the strong local space charge fields inevitably present oppose and 3,050,652 Patented Aug. 21, 1952 even check all emission of ions and electrons if they are ejected from separated sources.

For most eflicient operation of an ion propulsion system, the above described neutralization process must, therefore, insure that the total ion and electron currents from the vehicle be exactly equal in order that thrust be sustained. Also, neutralization must be achieved in a manner so that the initial spatial separation of electron and ion streams ejected by the vehicle is reduced to a minimum, producing an electrically neutral beam, 1n order to avoid wasteful power drain. This requires that the discharge velocity of the electrons match that of the ions.

It is an object of my invention to provide new and improved apparatus and methods employing a beam of high speed ions to develop a force for propulsion purposes m which the total electrical charge of the beam at its exit point is substantially zero.

It is another object of my invention to provide new and improved methods and apparatus for neutralizing the discharge of an ion propulsion system.

It is still another object of my invention to improve the efficiency of operation of an ion propulsion system.

One of the features of my invention consists in passing a beam of high speed ions utilized for force developing purposes through a drift space in which the ions encounter a cloud of electrons and thoroughly mingle therewith before emerging from the space and limiting the number of electrons emerging from the space to that required to neutralize the electrical charge of the ions.

In one of its forms, my ion beam force developing system employs two successive electrodes at substantially equal potentials, through which the ion beam is passed, and an arrangement to introduce electrons between the electrodes. The electrons so introduced are trapped in a potential well created by positive ion space charge between the electrodes in which they oscillate, but do not necessarily leave the vehicle. By neutralizing space charge in the region between electrodes, they prevent the formation of strong electric fields in this region, allowing other electrons emitted in the space to be drawn from the vehicle by the already emitted ion beam at low uniform potentials, thereby avoiding excessive drain of power.

The invention, both as to its organization and operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in which:

FIG. 1 is a schematic cross section of a portion of an ion propulsion system embodying my invention, and

FIGS. 26 are curves illustrating certain operational characteristics of the system of FIG. 1.

In the structure of FIG. 1, ions are emitted normally from the surface of a curved emitter l and pass through the aperture 2 in an accelerating electrode 3 into a field free region or drift space 4 defined by a neutralizing electrode 5 and a drift electrode 6. Electrodes 5 and 6 are maintained at substantially the same potential, which is intermediate the potential of emitter 1 and accelerating electrode 3. For example, electrodes 5 and 6 may be at zero potential, while emitter 1 is at a potential which is 2500 volts positive and electrode 3 at a potential 2500 volts negative with respect to electrodes 5 and 6. The ion beam would then receive a final velocity equal to that received in a direct potential fall of 2500 volts. Obviously, other values of either emitter or accelerating electrode potentials may be employed as dictated by the desired final ion energy.

Emitter 1 may be of the perfusing type and may comprise a louvered, sintered, or woven structure of a suitable metal, such as tungsten or tantalum, having a high work function. Vapors of an ionizable gas, such as cesium, are supplied to one surface of emitter 1 from a thermally insulated reservoir 7 containing metallic cesium through a duct 8. The ionizable vapor Within reservoir 7 is maintained at sufi'icient pressure to produce a desired rate of flow by a heater of a suitable material, illustrated as a resistance 9, to which heater current is supplied o ver leads 10. The emitter 1 is also maintained at high temperature by a similar heater, not shown. A cesium atom, upon striking the hot emitter material during its passage through the porous structure of emitter 1, becomes ionized so that a supply of cesium ions is provided from the porous emitter surface 1. The ions formed from cesium which has passed through the porous emitter 1 are accelerated by the difference in potential between the emitter electrode and the accelerating electrode 3 to form a beam of high speed ions. The shapes of emitter electrode 1 and accelerating electrode 3 are such that ions, which leave the emitter surface normally, move radially to pass through the aperture in accelerating electrode 3 into the field free region 4 where they fiow in parallel paths as a collimated beam until they exit through an aperture in electrode 6. Any suitable structure (not shown) may be employed to support the electrodes in fixed spaced relation.

As has been pointed out before, in utilizing an ion beam as a force developing device, particularly for propulsion purposes, it is essential that the beam be electrically neutral when it emerges from. the system to prevent rapid charging of the system and also to eliminate undesirable space charge fields. In order to produce such beam neutralization within the field free region defined by neutralizing electrode 5 and drift electrode 6, I provide means to introduce or inject into the region a controllable amount of electrons, illustrated in FIG. 1 as a cathode 11, supplied with heating current from a suitable potential source through a variable resistance 12.

FIGS. 2-5 illustrate possible potential distributions in the region between accelerator electrode 3 and drift electrode 6 before electrons are introduced into this region. A detailed theory of these types of potential distributions has been presented by C. E. Pay, A. L. Samuel and W. Shockley in Bell System Technical Journal 17, page 49 (1938), for the analogous case of electron beams in a vacuum tube. Thus, FIG. 2 represents the condition where at point 13 the potential is sufliciently positive that all ions are reflected and return to the accelerating electrode.

FIG. 3 illustrates a condition where the potential at point 14 is of such a value that a virtual anode is established. Some ions will be reflected and some will pass beyond this point. Obviously, if electrons can be introduced into this region, they will lower the potential to a value such that all ions will pass beyond the point and none will be reflected.

FIG. 4 illustrates a potential distribution in which all ions proceed through the region and electrons can be retained Within the region.

From an inspection of FIGS. 3 and 4, it is apparent that the potential distribution curve must pass through a maximum point, 14 or 15, of value not greater than the emitter potential, within the drift space defined by electrodes 5, 6 in order that electrons can be trapped in this drift space. Furthermore, the electrons emitted from a source at the potential of electrodes 5, 6 will be unable to move through aperture 2, so that they will not enter the region of ion acceleration and will not strike the ion emitter 1.

FIG. 5 illustrates the final condition, in which the potential in this region does not possess a maximum, so that electrons cannot be retained in the region 4 to neutralize the ion beam, but will, instead, pass through aperture 2 and strike the ion emitter 1. It can be shown that the desired condition for electron retention can always be achieved if the drift electrode 6 is at a potential 4 intermediate that of the accelerator electrode 3 and ion emitter It and, moreover, is located at suificient distance from accelerator electrode 3. The neutralizer electrode 5 is then properly positioned at the point where the potential again reaches that of electrode 6.

In accordance with my invention, I provide beam neutralization by establishing a stationary cloud of electrons in the region 4 between the neutralizing electrode 5 and the drift electrode 6. The electrons emitted from the cathode 11 are retained by positive ion space charge in the drift space 4. These electrons, when introduced into this region, exchange sufficient momentum in scattering collisions with the ions to insure trapping and, therefore, accumulate until they have completely neutralized the region. This region constitutes an electron reservoir from which current required to maintain the system neutrality is supplied. By regulating the electron emission from cathode H, such that it can replenish the electrons drawn from the drift region at a rate which equals the rate at which they leave the system, this region functions as a plane emission limited electron source and a steady state condition is achieved. In this steady state condition, the potential of the filled electron trap region remains slightly negative and electrons flow smoothly out with the ion beam.

The condition described above is illustrated in FIG. 6, in which line 1-1 illustrates the position of the emitter surface, line 33 the position of the accelerating electrode 3, line 55 the position of neutralizer electrode 5, line 6-6 the position of the drift electrode 6, and the potential at each point is given by the solid curve. Before the introduction of electrons, the potential between electrodes 5 and 6 has a maximum, as shown in FIG. 4, and is shown as a dashed line in FIG. 6. In the steady state neutralized condition, this region is field free because it contains a stationary cloud of electrons. In order that the drift space may retain electrons without the use of grids at its entrance and exit, the potential variation to the left of line 5--5 must be such that a potential barrier is established which extends completely across the aperture in electrode 3 of FIG. 1, and electrons are not drawn into the ion acceleration region between electrodes I and 3 of FIG. 1 since such action would remove electrons from the trap region and bombard the ion emitter. Similarly, the potential at electrode 6 must be such that an electron does not pass this point unless it accompanies an ion which it electrically neutralizes. It can be demonstrated mathematically that the scattering action of the electric field of each ion on the electrons is such as to make their motions while in region 4 become entirely random and insure their retention between electrodes 5 and 6 so that the electrons constitute a cloud of negative charges. It can also be demonstrated mathematicah ly that the attraction of the positively charged ions beyond electrode 6 will extract electrons from this cloud at Such a rate that a neutral ion beam is maintained beyond the electrode 6, provided that electrons so drawn from region 4 are exactly replenished by emission from cathode ill. Since the region within the electron reservoir, in which the stationary cloud of electrons exists, is substantially a field free region, ions entering the region retain the velocities with which they enter this neutralized region. When they leave the region to a point downstream of the ion beam, each ion is accompanied, on the average, by an electron so that the beam is electrically neutral. In addition, the electron achieves a velocity equal, on the average, to that of the ions, thereby minimizing the power required for a given thrust to be developed.

While in the foregoing a particular embodiment of this invention has been shown, it will, of course, be under stood that it is not limited thereto since many other modifications in the circuit arrangement and in the instrumentalities employed may be made. It is contemplated by the appended claims to cover any such modifications as fall within the true spirit and scope of this invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a force developing system, the combination comprising means providing a beam of high speed ions, means establishing a field free region through which said beam is projected, and means including a source of electrons for introducing electrons into said beam as it passes through said region at a rate such that the total electrical charge of the beam is substantially zero as it emerges from said region.

2. In a force developing system, the combination comprising means providing a beam of high speed ions, a pair of electrodes defining a space through which said beam is projected, means for injecting electrons into said space to establish therein a cloud of electrons of a density sufficient to neutralize the charge of the ions and form a field free region, the potential of said electrodes being the same and of a value relative to the potential of the beam that ions are decelerated as they enter the space and substantially all electrons are trapped in said space, the rate of injection of electrons into said space being such that the total electrical charge of the beam is substantially zero as it emerges from said space.

3. In a force developing system of the type employing a collimated beam of high speed ions, the method of neutralizing the electrical charge of the beam which comprises passing the beam through a region free of external fields, forming a cloud of electrons in the region through which the ions pass, injecting electrons into the region at a rate sufficient to substantially completely neutralize the electrical charge of the beam, and permitting only electrons utilized for neutralizing the beam to emerge from the region with the beam.

4. In an ion propulsion system, the combination comprising an emitter electrode, an accelerating electrode, a neutralizing electrode, and a drift electrode supported in fixed spaced relation, said neutralizing electrode and drift electrode being maintained at substantially the same potential whereby they define a drift space, means providing ions to said emitter electrode, said accelerating electrode being at a high negative potential relative to said emitter electrode whereby ions are accelerated to enter into said drift space, and means for introducing electrons into said drift space to neutralize the electrical charge of the ions within the space.

5. In an ion propulsion system comprising means for forming a collimated beam of high speed ions, a pair of electrodes having apertures therein through which said beam passes, said electrodes being maintained at substantially the same potential whereby they define a field free region through which said beam passes, and means for injecting electrons into said field free region substantially to neutralize the electrical charge of the beam, the rate of injecting electrons into said beam being such that the total electrical charge of the beam is substantially zero as it emerges from the region.

6. In an ion propulsion system employing a collimated beam of high speed ions, the method of neutralizing the electrical charge of the beam which comprises passing the beam through a region free of external fields, injecting electrons into the beam as it passes through said region, establishing a potential barrier across the entrance of the region so that electrons cannot freely leave the region, and controlling the rate of injecting electrons into the region so that only enough electrons enter the region to substantially completely neutralize the electrical charge of the beam.

7. In a force developing system, a first electrode, a second electrode having an aperture therein and arranged to be maintained at a negative potential with respect to the first electrode, means supplying an ionizable vapor to said first electrode whereby a high speed beam of ions is directed through said aperture, a pair of electrodes having apertures therein aligned with the aperture in said second electrode, means for introducing electrons into the region between said pair of electrodes, the potentials of said pair of electrodes relative to the potentials of said first and second electrodes being such that electrons introduced into said region are prevented from reaching said first and second electrodes and are trapped in said region to neutralize said beam, said beam as it leaves said region attracting electrons at a rate such that a neutral ion beam is maintained beyond said pair of electrodes, and means to control the rate of introducing electrons into said region to equal the rate at which electrons are attracted by the beam leaving said region.

References Cited in the file of this patent UNITED STATES PATENTS 2,880,337 Langmuir et al. Mar. 31, 1959 

